1. Field of the Invention
This invention relates to a thermal print head with resistive heating layers on a flexible organic resin film sheet.
2. Background of the Prior Art
Thermal print heads are installed in facsimile devices and the other similar recording devices. Advanced applications of such heads have presented needs for high precision and high speed recording of characters and images a thermally sensitive material, and compact heads for assembly in the various devices. Furthermore, in applications involving color printing apparatus which have a plurality of thermal heads for each color, the heads are required to exhibit thin widths to provide for parallel arrangement. Responding to such requirements, a rod type head structure provided with a metal member of circular or eliptical shape in cross-section was developed and coated with a glass glaze as illustrated in Japanese utility model laid-open No. 57-193545 and Japanese patent laid-open No. 58-92576. A resistive heating element array and metal circuit wiring connected thereto are therefore formed on the curved glass glaze surface. However, such a structure contains significant problems as follows: namely, the formation of the resitive heating film and metal circuit wiring usually use photo-etching technique, the technique comprising an exposure process including photo engraving with a photo resist mask on engraved film. To make the photo resist mask pattern, exposing light is projected on the photo resist layer through a pattern mask intimately contacted with or spaced apart from the resist layer. On exposure, if the distance between the mask pattern and photoresist layer is not constant for a portion of the exposed area, the high density pattern of resistive heating elements and metal circuit layers cannot be precisely formed. As a result, it is difficult to manufacture desired high quality thermal print heads.
To address the above problems, a flexible film type thermal print head has been proposed. The method of manufacturing comprises a first step of forming resistive heat layers on a flat flexible film and a second step of rolling and adhering the flexible film along a side surface of a rod substrate provided with a ridge. In practice, however, since the electrical resistance values of the resistive heating elements each, after adhering to the rod substrate, vary widely compared to the values before adhering, such thermal print heads cannot be practically manufactured.